Working memory maintenance is fundamental for the orderly pursuit of goals in the face of irrelevant, distracting stimuli. Both prefrontal and visual areas are thought to contribute to the maintenance of items in working memory, but the exact nature of the interaction between these areas has yet to be causally examined. This proposal combines neurophysiological recordings of single unit activity and local field potentials with pharmacological manipulations of the prefrontal cortex, to determine its contribution to memory-driven changes in neural activity and synchrony within visual areas. Local field potentials allow the examination of oscillatory signatures and phase locking, and their contribution to the strength of the visual representation. Such changes in oscillatory power and the timing of spikes relative to ongoing oscillations have recently been shown to reflect the content of working memory. Experiments in this proposal will 1) causally test whether prefrontal cortex drives these oscillations in posterior areas, 2) construct a model and test its predictions for how this top-down signal drives oscillatory changes, 3) determine how these changes alter the representation of visual stimuli presented during working memory maintenance, and 4) establish the relationship of these changes in oscillatory power and spike timing to the stimulus? ability to impact behavior. This fundamental understanding of the neural mechanisms contributing to working memory maintenance will in turn provide insight into the basis for working memory impairments in disorders such as schizophrenia, attention deficit hyperactivity disorder, and Parkinson?s disease.